In the processing of petroleum hydrocarbons and feedstocks such as crude oil and petroleum processing intermediates, and petrochemicals and petrochemical intermediates, e.g., gas, oils and reformer stocks, chlorinated hydrocarbons and olefin plant fluids such as deethanizer bottoms, the hydrocarbons are commonly processed at temperatures of 100.degree. to 1000.degree. F. Similarly, such petroleum hydrocarbons are frequently employed as heating mediums on the "hot side" of heating and heat exchange systems such as vacuum tower bottoms and slurry systems. In both instances, the petroleum hydrocarbon liquids are subjected to elevated temperatures which produce a separate phase known as fouling deposits, within the petroleum hydrocarbon. In all cases, these deposits are undesirable by-products. In many processes, the deposits reduce the bore of conduits and vessels to impede process throughput, impair thermal transfer, and clog filter screens, valves and traps. In the case of heat exchange systems, the deposits form an insulating layer upon the available surfaces to restrict heat transfer and necessitate frequent shutdowns for cleaning. Moreover, these deposits reduce throughput, which, of course, results in a loss of capacity with a drastic effect in the yield of finished product. Accordingly, these deposits have caused considerable concern to the industry.
Organic foulants are usually higher molecular weight materials ranging in consistency from that of tar to rubber to "popcorn" to "coke". The exact composition of such foulants is difficult to identify.
One particularly troublesome type of organic fouling is caused by the formation of polymers that are insoluble in the hydrocarbon or petrochemical fluid being processed. The polymers are usually formed by reactions of unsaturated hydrocarbons, although any hydrocarbon can polymerize. Generally, olefins tend to polymerize more readily than aromatics, which in turn polymerize more readily than paraffins. Trace organic materials containing hetero atoms such as nitrogen, oxygen and sulfur also contribute to polymerization.
Polymers are formed by free radical chain reactions. These reactions, shown below, consist of two phases, an initiation phase and a propagation phase. In reaction 1, the chain initiation reaction, a free radical represented by R.multidot., is formed (the symbol R can be any hydrocarbon). These free radicals, which have an odd electron, act as chain carriers. During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see reaction 2c), forming the unwanted polymers which accumulate on heat transfer surfaces.
Chain reactions can be triggered in several ways. In reaction 1, heat starts the chain. Example: when a reactive molecule such as an olefin or a diolefin is heated, a free radical is produced. Another way a chain reaction starts is shown in reaction 3, where metal ions initiate free radical formation. Accelerating polymerization by oxygen and metals can be seen by reviewing reactions 2 and 3.
1. Chain Initiation
R--H.fwdarw.R.multidot.+H PA1 a. R.multidot.+O.sub.2 .fwdarw.R--O--O.multidot. PA1 b. R--O--O.multidot.+R'-H.fwdarw.R'.multidot.+R--O--O--H PA1 c. R.multidot.+C.dbd.C.fwdarw.R--C--C.multidot..fwdarw.polymer PA1 a. Me.sup.++ +RH.fwdarw.Me.sup.+ +R.multidot.+H.sup.+ PA1 b. Me.sup.++ +R--O--O--H.fwdarw.Me.sup.+ +R--O--O.multidot.+H.sup.+ PA1 a. R.multidot.+R'.multidot..fwdarw.R--R' PA1 b. R.multidot.+R--O--O.multidot..fwdarw.R--O--O--R
2. Chain Propagation
3. Chain Initiation
4. Chain Termination
Research indicates that even very small amounts of oxygen can cause or accelerate polymerization. Accordingly, to inhibit this insidious fouling problem, it is highly desirable to provide a polyfunctional process antifoulant which can, among other functions, inhibit oxygen based polymerization initiation. This antioxidant function serves as a "chain-stopper" by forming inert molecules with the oxidized free radical hydrocarbons, in accordance with the following reaction:
Chain Termination EQU ROO.multidot.+Antioxidant.fwdarw.ROOH+Antioxidant.multidot.(--H)
In addition to the desirability of inhibiting oxygen based polymerization, it is highly desirable to inhibit the catalytic formation of gums and other deposits, which are caused by metallic impurities, such as copper and/or iron, which may be present in the process fluids. These types of antifoulants are referred to as "metals coordinators" or "metal deactivators" and function by the formation of a complex or ligand with the metallic impurity in the process fluid.
Unlike organic deposits, inorganic deposits can be simple to identify. One example is ammonium chloride formed as the reaction product of injected ammonia in a crude overhead system. Other inorganic deposits include e.g., metallic salts, oxides, sulfides, etc. of iron, copper and vanadium. Such deposits may be present in the original feed as "ash" or they may be the result of corrosion or precipitation in equipment where fouling is evident. In some cases, fouling and corrosion may be related in that solving the corrosion problem which exists upstream may improve the downstream fouling problem.
As to the problem of corrosion, the root of this problem is usually attributed to HCl or H.sub.2 S contamination of the aqueous phase that is entrained in the process fluid. Other acids such as carboxylic and carbonic acids may also exacerbate the corrosion problem.
Corrosive attack on the metals normally used in the low temperature sections of a refinery processing system, i.e., where water is present below its dew point, is an electrochemical reaction, generally in the form of acid attack on active metals as shown in equation 1. EQU At the anode Fe.fwdarw.Fe.sup.++ +2(e) (1) EQU At the cathode 2H.sup.+ +.multidot.2(e).fwdarw.2H (2) EQU 2H.fwdarw.H.sub.2 ( 2a)
Equation 2 expresses the reduction of hydrogen ions to atomic hydrogen. The rate of the cathodic reaction generally controls the overall corrosion rate.
The aqueous phase is simply water entrained in the hydrocarbons being processed and/or water added to the process for such purposes as steam stripping. Acidity of the condensed water is due to dissolved acids in the condensate, principally HCl and H.sub.2 S. The HCl is formed by hydrolysis of calcium and magnesium chlorides originally present in the brines produced concomitantly with the hydrocarbons--oil, gas, condensates.
The bulk of these brines are separated at the field production facilities. Most of what remains after field separation is removed by desalting equipment at the refinery, upstream of the crude still and subsequent processing units.
Even under the best of conditions, however, a small amount of salt, several pounds per thousand barrels of charge (ptb expressed as NaCl) will reach the crude still. A portion of this, principally the chlorides of calcium and magnesium, decompose to give gaseous HCl which dissolves in overhead condensates and forces the pH down as low as 2 to 3, which is severely corrosive to steel equipment. With sour crudes, an additional corrodent is H.sub.2 S, either originally present in the sour oil, gas or condensate production and/or formed at processing temperatures by decomposition of sulfur compounds in the charge stocks. Certain sulfur compounds, such as low-molecular-weight, water-soluble mercaptans may also be corrodents.
There are many areas in the hydrocarbon processing industry where antifoulants have been used successfully; the main treatment areas are discussed below.
In a refinery, the crude unit has been the focus of attention, primarily because fuel use directly impacts on processing costs. Antifoulants have been successfully applied at the exchangers; downstream and upstream of the desalter, on the product side of the preheat train, on both sides of the desalter makeup water exchanger, and at the sour water stripper.
Hydrodesulfurization units of all types experience preheat fouling problems. Among those that have been successfully treated are reformer pretreaters processing both straight run and coker naphtha, desulfurizers processing catalytically cracked and coker gas oils, and distillate hydrotreaters. In one case, fouling of a Unifiner stripper column was solved by applying a corrosion inhibitor upstream of the problem source.
Unsaturated and saturated gas plants (refinery vapor recovery units) experience fouling in the various fractionation columns, reboilers and compressors. In some cases, a corrosion control program along with the antifoulant program gave the best results. In other cases, antifoulants alone were enough to solve the problem.
Cat cracker preheat exchanger fouling, both at the vacuum column and at the cat cracker itself, has also been corrected by the use of antifoulants.
In heavy oil treating and cracking units, fouling of preheat trains of the vacuum tower bottoms feedstock has been successfully reduced by antifoulants.
In petrochemical plants, the two most prevalent areas for fouling problems are ethylene and styrene plants. In an ethylene plant, the furnace gas compressors, the various fractionating columns and reboilers are subject to fouling.
In butadiene plants, absorption oil fouling and distillation column and reboiler fouling have been corrected with various types of antifoulants.
Chlorinated hydrocarbon plants, such as VCM, EDC and perchloroethane and trichloroethane have also experienced various types of fouling problems.
The present invention is directed toward methods using multifunctional antifoulant compositions which are useful in controlling fouling encountered in the petroleum and petrochemical systems above-identified. More specifically, these methods and compositions, due to their multifunctional characteristics, may be applied effectively to inhibit fouling caused by oxygen-based free radical formation, metal catalysis, corrosion and polymer aggregation.